Sensitivity analysis of bi-metal stacked-gate-oxide hetero-juncture tunnel fet with Si0.6Ge0.4 source biosensor considering non-ideal factors

This article provides insights in designing a dielectrically modulated biosensor by adopting high-k stacked gate oxide proposition in a bi-metal hetero-juncture Tunnel Field Effect Transistor (BM-SO-HTFET) with Si0.6Ge0.4 source. The integrated effect of heterojunction and stacked gate oxide leads to enhanced electrical performance of the proposed device in terms of carrier mobility and suppressed leakage current. Nano-cavity engraved beneath the bi-metal gate structure across the source/channel end acts the binding site of the biomolecules to be detected. This Configuration leads to improved control of biomolecules over source/channel tunnelling rate and the same is reflected in the sensing ability of the device while extracting the ON current sensitivity (SON) of the sensor. The reported biosensor is simulated using Silvaco ATLAS calibrated simulation framework. The analysis of the device sensitivity is carried out varying dielectric constants (k) of various biomolecules, both neutral as well as charged. Our study reveals that BM-SO-HTFET with Ge mole fraction composition x = 0.4 exhibits sensitivity as high as 4.1 × 1010 for neutral biomolecules and 3.2 × 1011 for positively charged biomolecules with k = 12. Furthermore, a transient response profile for the drain current with various biomolecules is explored to determine the varying settling time. From the simulation results, it is noted that BM-SO-HTFET exhibits ON current sensitivity of 4.1 × 1010 and 3.2 × 1011 for neutral and charged biomolecules respectively. In addition to this, for highly sensitive and real time detection of biomolecules, the impact of temperature and certain non-ideal factors drifting from ideal case of fully filled cavity have also been considered to analyze its optimum sensing performance.


Introduction
Present day microbiological progress has steered the researchers to a focused and streamlined path in biosensing performance investigations.Biological molecules or biomolecules or bio-species are fundamental ecological components.DNA (Deoxyribonucleic acid), Biotin, Streptavidin, Gluten, Zenin, Keratin, Gelatin play critical roles in lifeforms.Field Effect Transistors (FET) based lab-on-chip biosensors for efficient and accurate label free detection of biomolecules have emerged as a major interest in the research arena for both ideal and non-ideal issues [1,2].The complex lab based expensive sensors can be substituted by FET based biosensors which have easy fabrication steps, higher sensitivity, higher lifetime, high reliability, low power consumption and low cost [3].To obtain the best performance of a FET based biosensor it should be highly sensitive.Ion sensing FET (ISFET) had become a prime focus for researchers in 1970s for sensing biomolecules with charge density [4].But as reported in [5], ISFET could not detect neutral biomolecules.The above problem has been solved with the introduction of dielectrically modulated FET based biosensors with the potential of sensing neutral biomolecules as well as biomolecules with charge density.In dielectrically modulated FET based biosensors, nanogap engineering is implemented under the gate metal by wet etching the gate oxide [6].The biomolecules are selectively captured with the help of receptors in the engineered nanogap.With the applied gate bias, the effectual gate-channel capacitance varies with different dielectric constant values of different biomolecules localized inside the engineered nanogap giving rise to readable signals in the form of drain characteristics, transfer characteristics, channel conductance, threshold voltage characteristics.The dielectric constant values vary with varying biomolecules such as Streptavidin, Biotin, 3-Aminopropyl Triethoxysilane (APTES), Gluten, Zenin, Keratin, Gelatin having k = 2.1, 2.63, 3.57, 5, 7, 10, 12 respectively [7].The integration of dielectrically modulated n-FET and p-FET based biosensors with Complementary Metal Oxide Semiconductor (CMOS) architecture provides satisfactory compatibility for further logic applications.Bulk Metal Oxide Semiconductor Field Effect Transistor (MOSFET) emerged as a suitable candidate for dielectrically modulated biosensing [8].But in accordance with Moore's Law, the downscaling of technology nodes causes MOSFETs to pose several problems.It's limitation to 60 mV/dec Subthreshold Swing (SS) at ambient conditions, giving rise to Short Channel Effects (SCEs), KT/q limit, increase in leakage current, high static power consumption makes MOSFET based biosensors not suitable for efficient sensing of biomolecules as well as long response time [9,10].Dielectrically modulated Tunnel Field Effect transistors (DM-TFETs) based biosensors in recent times are under extensive research due to their capability of overcoming the above-mentioned limitations with MOSFET based biosensors [11].
TFET is also ideal for low power Very Large-Scale Integration (VLSI) utilities because of their significantly low OFF current therefore having reduced static power consumption [12].DM-TFET based biosensors provides sub-60 mV/dec SS and faster response time.The Bandto-Band Tunnelling (BTBT) phenomenon of TFET based biosensors makes them essentially important candidates for biosensing operations with dielectric modulation approach [13,14].Recently, Covid-19 illness causing virus, SARS-CoV-2 has come under tremendous attention due be-cause of the pandemic break-out that has taken countless lives and continues as a threat and demands fast and accurate detection [15].It is reported in [3] that SARS-CoV-2 virus can be detected by TFET based biosensors.Besides the advantages of TFET based biosensors over MOSFETs, TFETs have extremely low ON current (I ON ) and ambipolar behaviour for conductivity which can lead to poor performance and even circuit failure [16].Researchers are working to find out the best possible architecture that can increase the I ON as well as reduce the ambipolar current behaviour.As reported in [17], asymmetric doping profiles of source and drain can suppress ambipolar current behaviour and the I ON can be improved by sharp source doping, thin silicon body and a double gate architecture that will improve the gate controllability on the channel charge.The Z-shaped gate hetero dielectric horizontal source pocket TFET (ZHP-DM-TFET) based biosensor [18], Gate All Around TFET (GAA-TFET) [19], SiGe Source and Pocket-Doped Channel [1], N+ Pocket Doped Vertical TFET Based Dielectric-Modulated Bio-sensor [20], Dielectric Modulated Dual Channel Trench Gate TFET-Based Bio-sensor [21], Back-Gate Bias and Front-Gate Engineering DMTFET-Based Bio-sensors [22] focuses on improving the ON current sensitivity by improving the carrier mobility.Lately, tunnel FETs with integrated hetero-juncture architecture have grabbed attention [23].The mindful selection of bandgap modulating materials at the source side allows improvement in ON-state current as well as the sub-threshold slope (SS) by improving the tunnelling rate.Therefore, in terms of considering ON-state current and SS as a sensitivity parameter, heterojuncture architecture in TFETs is of real advantage.However, hetero-juncture architecture also leads to an increase in the sub-threshold current which is an issue in terms of power consumption [24].Therefore, further research is needed to improve the hetero-juncture architecture in TFETs.
In this work, a Bi-Metal Stacked-oxide Hetero-juncture Tunnel Field Effect Transistor (BM-SO-HTFET) dielectrically modulated biosensor device has been utilized with an incorporated Si 0.6 Ge 0.4 source.The engineered gate oxide architecture facilitates the carrier mobility by mitigating the leakage current.Bi-gate metal structure improves tunnelling at the sourcechannel interface thereby improving the ON current and higher control over the channel charge [14].This delves deeper into the sensing abilities of the BM-SO-HTFET dielectrically modulated biosensor with various biomolecules consisting of dielectric constant values ranging from 5 to 12.The biosensor device exhibits ON current sensitivity for neutral biomolecules (SON = 4.1 × 10 10 at k = 12), charged biomolecules (SON = 3.2 × 10 11 at k = 12) for fully filled nanogap at ambient temperature.Furthermore, real-time bio-sensing performance considering non-ideal nanogap filling is extensively investigated in terms of ON current sensitivity.The simulated BM-SO-HTFET has a high-k gate stack that mitigates the leakage current subsequently improving I ON , while also being thermodynamically stable [25,26].Bi-metal with work-function of tunnelling gate, ϕ tg = 3.8eV and work-function of auxiliary gate, ϕ ax = 4.8eV is utilized for constructing the gate architecture [2].To emulate biosensing, the nanogap is considered air-padded at k = 1.At air-padded nanogap (k = 1), the effective gate-channel capacitance becomes less influential as compared to biomolecules padded nanogap thereby mitigating effective tunnelling.Therefore, a metal with lesser work-function (ϕ tg = 3.8 eV) positioned over the nanogap to overcome this is-sue.It is known that a filled nanogap will enhance the tunnelling vertical electric field leading to acute band bending and increased tunnelling.Metal with increased work-function (ϕ ax = 4.8 eV) utilized over high-κ insulator, HfO 2 to accurately facilitate charge carrier tunnelling for padded nanogap.Therefore, tunnelling gate controls subthreshold swing (SS) while auxiliary gate governs the leakage current [2].From reported articles it is known that mono nanogap biosensors exhibit improved outputs in comparison to mirrored nanogap biosensors due to their ability to impregnate acute band-bending at the tunnelling interface for various biomolecules simultaneously providing improved coverage area of the nanogap.The split nanogap architecture in mirrored nanogap devices reduces the coverage area of the nanogap thereby degrading effective gate to channel capacitance [2].

Device architecture and simulation framework
BM-SO-HTFET biosensor measurements are presented in Table 1.The simulation of the BM-SO-HTFET is carried out with SILVACO ATLAS commercial TCAD tool [27].The Wentzel-Kramer-Brillouin (WKB) approximation based non-local BTBT model has been implemented to incorporate effective tunnelling due to band-bending at the tunnelling juncture.CONMOB and FLDMOB models are used for mobility degradation.Furthermore, the classical Drift Diffusion model along with Shockley-Read-Hall (SRH) model and Auger model has been incorporated for facilitation of carrier generation and recombination.The Band Gap Nar-rowing model (BGN) and Fermi-Dirac statistics were further included to procure accurate band-to-band tunnelling.The calibration of the mole fraction parameter (x) has been done with the experimental results reported in [28] and implemented in the SILVACO ATLAS TCAD tool.The experimental results of the device transfer characteristics of SiGe PIN TFET [29] are fitted with the simulation results in terms of physical models and device specifications.The calibration results are highlighted in Fig 2.

Simulation results and discussions
A Si 1-x Ge x , a bandgap tuneable alloy semiconductor, is incorporated in the P ++ type source of BM-SO-HTFET dielectrically modulated biosensor.The mole fraction composition of Ge is Here, C is a factor which is a function of electron effective mass, B representing tunnelling possibilities.Transitsion constant is represented by σ.Electric field is represented by E. The tunnelling possibility in tunnel FET devices are mainly governed by the Wentzel-Kramer- Brillouin approximation (WKB) [21].
ffi ffi ffi ffi ffi ffi ffi ffi 2m * p ffi ffi ffi ffi ffi Here, λ and m* represents the screening tunnelling span and the electron effective mass respectively.The semiconductor forbidden gap (bandgap) is represented by E g , 4ϕ represents the tunnelling range in terms of energy.From (1), it can be noticed that the acute band-bending can be achieved with increased electric field.From (2), it is deduced that as the k values rise, the tunnelling span narrows and T WKB surges thereby increasing drive current/ON current [30].
Fig 6A demonstrates the device transfer characteristics (I DS -V GS ) of the BM-SO-HTFET for increasing dielectric constant (k) values of the localized biomolecules in the nanogap.It is noted that air-filled nanogap yields poor I ON ~10 −17 A/μm [31].However, I ON increases significantly with rising k values whereas maintaining an appreciably low I OFF which improves ON-OFF switching.SNR, signal-noise-ratio (SNR) gets hindered in real time for these FET based biosensors [32].Low I OFF current results in reduced consumption of static power in traditional Bi-Metal TFET biosensor.Furthermore, researchers are implementing a transient response approach for exploiting the settling time of I ON with different biomolecules inside the nanogap [32].Biosensing parameters like sensitivity and selectivity for a certain applied gate voltage can be meticulously studied using transient response of a FET based biosensor device.Fig 6B shows the transient response of BM-SO-HTFET biosensor localizing different biomolecules inside the nanogap.It is evident from the analysis that as the k values for different biomolecules inside the nanogap increase, the I ON increases and reaches a stable state at different settling times [30].In the case of gelatin biomolecule (k = 12), I ON takes the least settling time to reach transient stable state since vertical electric field induces acute band bending at the tunnelling junction due to higher dielectric constant of the biomolecule [30].Whereas, for an empty nanogap (k = 1), prolonged settling time is observed to reach transient stable state as air-filled nanogap mitigates band bending at the tunnelling juncture, thereby reducing BTBT rate.

Sensitivity of ON current (S ON )
The sensitivity of the biosensor is one of its most important metrics to determine the biosensing functionality.Various sensitivity profiles like threshold voltage sensitivity, I ON /I OFF sensitivity, ON current sensitivity can be considered to analyse the performance of a biosensor.However, in this work, we consider sensitivity of ON current for BM-SO-HTFET biosensor as shown in (3).S ON is defined by the fraction of ON current when the biomolecules are localized and absent in the nanogap.I ON has been selectively considered to determine sensing performance since in TFETs I ON depends on BTBT rate.BTBT however, is independent of temperature.Subthreshold Swing (SS), threshold voltage shift (4Vth) and I ON /I OFF ratio depend heavily on thermionic emission phenomenon [2].Hence temperature dependence of these parameters might hamper the sensing functionality of the biosensor.The dependence of sensitivity on temperature is elaborated under section 3.2.Asymmetrical doping of source (1 × 10 20 cm -3 ) and drain (1 × 10 18 cm -3 ) achieves better sensitivity of the TFET based biosensors [33].

Impact of temperature on S ON
The sensitivity profile of dielectrically modulated biosensors can be furthermore investigated considering its variation from ambient temperature.It is evident from Fig 8A that I OFF increases with increase in temperature as recombination rate increases in BM-SO-HTFET biosensor for k = 7 and k = 12 [34].Insignificant change in I ON at higher gate voltages is observed.Sensitivity varies with gate voltage and temperature.The I OFF keeps on increasing in the presence of air in the nanogap with varying temperature as a result the ON current sensitivity (S ON ) monotonically reduces as temperature rises, same can be observed in Fig 8B for biomolecules with dielectric constant (k = 7 &12).

Impact of charge biomolecules on S ON
The relation between the gate voltage and the biomolecule charge density (N) given in (4) represents the field effect transistor voltage balance equation.Where, C s represents surface potential, N represents biomolecules charge density, C eff represents the effective capacitance per unit area, ϕ ms represents the metal-semiconductor work function.There will be increased C s as +N increments at fixed gate voltage (V GS ) as shown in ( 4).This will result in increased sensitivity of BM-SO-HTFET biosensor [21].Increment in sensitivity takes place as the þqN C ef f component at HfO 2 increases depleting the intrinsic channel resulting in acute band bending at the tunnelling junction [33].On the other hand, as -N increases, C s reduces at fixed V GS that results in hindered device sensitivity.

Impact of non-ideal issues on S ON
This section broadly discusses the real-time manifestation of the biomolecule localization inside the engineered nanogap of the BM-SO-HTFET biosensor, drifting from the ideal case of fully filled nanogap [30,37].acteristics for all the four cases.It is noted that Case A exhibits the highest ON current and almost provides similar behaviour for a β = 100% filled nanogap because of the presence of gelatin at the source-channel tunnelling juncture that facilitates utmost BTBT thereby increased band bending resulting in enhanced ON current.However, for both Case B and Case C it is observed that low ON current is obtained since gelatin is located away from the tunnelling juncture which reduces the gate control and thus hinders BTBT at the same.Even though there is 50% air filled from the top in Case D, the complete localization of gelatin from the tunnelling juncture to the channel-drain interface leads to increased ON current than Case B and Case C.However, the air fill in Case D reduces its BTBT when compared to Case A. Subsequently, the ON current sensitivity is evaluated in Fig 10C .A maximum S ON = 3.8 × 10 11 is obtained at gelatin with β = 50% for Case A and a minimum S ON = 2.66 × 10 6 for Case C. Thus, it is understood that in real-time the presence of biomolecules at the source-channel tunnelling juncture even at β = 50% can provide optimum sensing performance.Furthermore, the real-time biosensor performance is even widely investigated by stacking the biomolecules inside the engineered nanogap [37] 2 shows the benchmarking of the  BM-SO-HTFET biosensor considering its S ON performance with the existing FET based biosensors.It is reported that the BM-SO-HTFET biosensor exhibits improved ON current sensitivity for neutral biomolecules than almost all the existing biosensors reported in the literature.

Conclusions
In this paper, a high-k stacked gate with bi-metal hetero-juncture TFET is proposed for label free biosensing applications.This study considers the effect of dielectric modulation within the nanocavity beneath the bi-metal gate structure that acts as the sensing site for the biomolecules.The change in dielectric constant (k) and charge densities of the bio-species alter the gate capacitance modulating the device characteristic trend.Hence, the impact of biomolecule immobilization at the binding sites have been studied on band-to-band generation rate, electric field and drain current characteristics of the device.Finally, the ON current sensitivity (S ON ) of the device is extracted which is utilized as the major sensing metric throughout the work.A better sensitivity (S ON = 3.2 × 10 11 ) is reported with Ge content x = 0.4 for positively charged biomolecules in comparison to neutral ones (S ON = 4.1 × 10 10 ).The impact of temperature variation depicts a reduction in S ON monotonically with rise in temperature.Moreover, some non-ideal cases are also considered to evaluate the detection limit of the device.From the results, it can be predicted that the reported structure can exhibit optimum sensitivity of S ON = 3.8 × 10 11 with biomolecules across the source-channel interface even with 50% fill factor.Finally, sensitivity is also determined for stacked biomolecules depicting S ON = 7.8 × 10 11 for completely stacked biomolecules (k = 5+7+10) inside the nanogap.Comparative analysis of the ON current sensitivity of BM-SO-HTFET biosensor with some of the reported FET based sensors pledges for the potential of the proposed structure for future ultra-sensitive applications.

Fig 1 (
Fig 1(A) presents the 2-D illustration of BM-SO-HTFET dielectrically modulated biosensor.Si 0.6 Ge 0.4 has been incorporated in the source with a highly P-type doping profile over the mildly (intrinsic) doped silicon channel forming a hetero juncture.A 40 nm channel length is considered.The SiO 2 (low-κ insulator) depth is 1 nm whereas HfO 2 depth is 2 nm.A nanogap of 30 nm distance and body depth of 10 nm is considered.The Si0.6Ge0.4source is 1 × 10 20 cm-3 doped; silicon drain is 1 × 10 18 cm-3 doped.The silicon channel is 1 × 10 15 cm-3 lightly determined by mole fraction (x) values.From Fig3, it is noted that as the value of x increases, it is observed that the bandgap of Si 1-x Ge x narrows till x = 0.4 for both empty and filled nanogap, thereby boosting the band-to-band tunnelling rate, GBTBT[29].However, the GBTBT doesn't improve at x = 0.6 and x = 0.8 values thereby making x = 0.4 a suitable choice for the simulation of BM-SO-HTFET.Fig 4A highlights increment of the tunnelling vertical electrical field profile.It is observed that vertical electric field is greatest at k = 12 (2.4 × 106 V/cm) that can be attributed by the increased gate coupling effect.Fig 4B represents that the Band-to-band tunnelling generation rate (GBTBT) that increments at greater k values (28 × 1030 cm-3s-1 at k = 12).The non-local Band-to-band tunnelling model activates the BTBT dynamics at the tunnelling juncture of the biosensor device [30].The Energy Band diagram profile shown in Fig 5A highlights the acute band-bending at the tunnelling interface for increasing dielectric constants of the biomolecules that is mainly attributed by the ϕ tg whereas, OFF current (I OFF ) is governed by ϕ ax .Thereby, enhancing the BTBT generation rate (G BTBT ) resulting in enhanced Ion values.Fig 5B represents the biosensor device threshold voltage profile for increasing dielectric constants of the biomolecules.As observed from the energy band diagram profile that the band gap narrowing with increasing dielectric constant of the biomolecules leads to increased vertical electric field thereby increasing ON current resulting in the decreased threshold voltage at greater dielectric constants.

Fig 5 .Fig 6 .
Fig 5. (a) Energy band diagram profile (b) Threshold voltage profile of the BM-SO-HTFET with increasing dielectric constant (k) values of the biomolecules.https://doi.org/10.1371/journal.pone.0301479.g005 Fig 7A highlights steep rise of S ON for increasing k values and it can be noted the k = 12, gelatin yields the maximum sensitivity, S ON = 4.1 × 10 10 .Fig 7B highlights the S ON profile as a function of gate voltage.It is observed that peak S ON (6.4 × 10 10 at k = 12) for increasing dielectric constant of biomolecules is achieved at ~1.35 V gate voltage.

Fig 10 .
Fig 10.(a) 2D illustration of BM-SO-HTFET biosensor with four different cases of gelatin fill factor (b) transfer characteristics of four different cases (c) ON current sensitivity for four different cases of gelatin fill factor.https://doi.org/10.1371/journal.pone.0301479.g010